Electric arc seismic source



Feb. 13,1968 C.O.BERGLUND ETAL 3,368,643

ELECTRIC ARC SEISMIC SOURCE Filed Oct. 19, 1966 TIMER- CONTROL STORED ELECTRICAL 5 ENERGY SOURCE 18 2e BRAKE 'B 22/ REEL J C 34) j 32 MOTOR 1 k I M 30 WATER Ki 4 WRE INLET ig-2 1 l2 I8 INVENTORS 53 CARL O. BERGLUND' v A. c. HILL ATTORNEYS United States Patent Ofilice 3,368,643 Patented Feb. 13, 1968 3,368,643 ELECTRIC ARC SEISMIC SOURCE Carl 0. Berglund, Dallas, and A. C. Hill, Richardson, Tex., assignors to Teledyne Industries, Inc., Geotech Division, a corporation of California Filed Oct. 19,1966, Ser. No. 587,741 6 Claims. (Cl. 181.5)

ABSTRACT OF THE DISCLOSURE An underwater acoustical source for efficiently convert ing electrical energy discharged between spaced immersed electrodes from a storage device into an intense plasma discharge to form a large steam bubble which subsequently collapses to provide an acoustical disturbance having an improved low-frequency content. The described embodiment includes means for pumping a stream of water through a hose and jetting the stream from one electrode toward and against the other. A fine wire is introduced through a T-joint at the pump end of the hose, and the stream entrains the Wire pulling it through the hose, out through the jet at one electrode, and sweeping it into contact with the other electrode to form a metallized path of higher initial conductivity. A very high peak current is then discharged through the path to vaporize it.

This invention relates to improvements in apparatus for generating underwater acoustical impulses, and more particularly relates to improved apparatus for increasing the efficiency of conversion of electrical energy to acoustical energy while decreasing the fundamental frequency thereof, especially in the type of seismic survey equipment employing a high-energy discharge through "water to set up a plasma bubble therein.

The prior art has suggested a number of different basic approaches to the problem of providing underwater seismic impulses, including explosions, magnetostrictive disturbances electrical discharge disturbances, etc. It is to the latter type of approach that the improvements of the present invention are directed. The electrical discharge usually is from a source of stored electrical energy which can be discharged suddenly between two electrode means immersed in the water, and usually towed behind a vessel. The current from the source is introduced at one of the electrodes, passes through and ionizes the water in a region between the electrodes, and is returned to the source through another electrode which is usually located relatively close to the first electrode. The source of stored energy is advantageously of the capacitive type which is triggered by suitable means to apply great energy across the electrodes. The passage through the water of this energy ionizes the water in said region and creates conductive ions whose character depends upon the available constituents in the water, for instance C1 A large steam bubble is formed by the heat generated at the plasma discharge in the region between the electrodes and the bubble is sustained beyond the interval of time that the discharge current is passing therethrough, depending upon the time required to dissipate the heat in the bubble region. The size of the bubble and its rate of change determine the content of the accoustical shock wave, and in turn this size depends upon the amount of electrical energy per unit time that can be passed through the interelectrode region. Moreover, the duration of the bubble is the major contributing factor in the determination of the fundamental frequency of the seismic shock wave. Thelower the frequency the better the penetration into bottom formations, and therefore it is desirable to create as large a bubble of as long a time-duration as possible.

The principal obstacle in the way of creating large bubbles of relatively long duration resides in the electrical resistivity of the discharge path, even after plasma is established in the region between the electrodes. The total resistance of the discharge path is the sum of the resistance of the conductors leading to the electrodes plus the resistance-of the path between the electrodes. The resistance of the current path in salt water never decreases into the milliohm range, and, in practical electrode configurations where the electrodes are separated by about a foot, the resistance is of the order of one ohm or more.

The improvement according to the present novel technique involves the introduction of metallic Wire across the region between the electrodes prior to each energy discharge, and further involves an improved-electrode configuration designed to ensure contact of the wire with the electrodes due to the manner in which the water flows across the electrodes. As a result of the energy discharged, =for example, using a 25,000 Joule source, bubble times of 30 to 35 milliseconds duration are typical, and this duration is approximately twice as long as is obtained using prior-art techniques without the addition of conductive ions to the plasma region. The present technique employs a repeating cycle wherein, after each discharge through the plasma region, the wire is replaced prior to the next discharge and during the interval of time when geophone arrays are receiving reflection data. For instance, the discharge of a new impulse may take place at intervals of four to six seconds.

It is the principal object of the present invention to provide for introducing small-gauge wire in the path between electrodes having substantial spacings for the purpose of reducing the resistance of the path in order to obtain larger bubble sizes. The path is metalized by the wire, which vaporizes as the electrical energy is dis charged through it, and thus aids the fiow of much higher peak currents. Greater discharge current has the effect of significantly increasing the efficiency of the accoustical source, i.e., the percent of electrical energy transformed into accoustical energy. Since substantial electrode spacings can be employed with the present technique, and since a longer bubble time also results. a much higher-pressure accoustical impulse containing a lower fundamental frequency is generated in the water. This greater energy is particularly useful for the purpose of obtaining seismic reflection data from geological beds located, for instance, several miles below the water bottom.

It is another object of this invention to provide pathmetalizing apparatus which facilitates the discharge of current between electrodes having substantial mutual spacings. Without such metalizing, widely spaced electrodes will not attain adequate ionization therebetween. Moreover, larger spacing contributes to greater electrode life, since closely-spaced electrodes are eroded away more quickly than wider-spaced electrodes passing the same electrical energy.

Still another object of this invention is to provide a system for creating an accoustical disturbance by discharging a large store of electrical energy between spaced electrodes in a nonconductive fluid medium. The technique of metalizing the path between electrodes makes it practical to use the present accoustical source in fresh water, for example, whereas it has formerly been considered useful only in a salt water environment. The size of the bubble whose collapse creates the accoustical shock wave depends upon the power of the electrical energy discharge creating it, and in fresh water there was simply not enough conductivity to establish a high-powered discharge. The metalizing of the path between electrodes provides the necessary initial conductivity to permit the flow of enough energy through the path to generate the desired steam bubble.

Yet another object of the invention is to provide an efficient working system wherein most of the path-metalizing equipment is located safely aboard the towing vessel so that it does not experience the destructive forces occurring every few seconds in the vicinity of the discharge region. Great attention has been directed toward providing an electrode array which is capable of withstanding these forces for a reasonable length of time and which is, to a limited extent, self-protective. Even heavy brass electrodes last only a few hours before they must be replaced. Under these conditions, it is very advantageous to be able to place the path-metalizing mechanism in a safe place, substantially inboard of the towing vessel.

Other objects and advantages of the present improved method and apparatus will become apparent during the following discussion of the drawing, wherein:

FIG. 1 is a pictorial view showing a vessel engaged in towing apparatus for generating an underwater accoustic impulse by an electrical discharge through the 'water;

FIG. 2 is a diagram illustrating apparatus for feeding metal wire into the region between electrodes; and

FIG. 3 is a perspective view of an electrode structure including Wire feeding means and a towing yoke.

Referring now to the drawing, FIG. 1 shows a vessel V moving across a body of water, the vessel carrying on board a source S of electrical energy connected by a cable C to the terminals A and B of two electrodes 12 and 14. The vessel V can also carry on board seismic recording equipment connected by another cable to a suitable geophone streamer (the latter equipment not being illustrated in the present drawing). Alternatively, such a geophone array can be towed behind a different vessel, all of these techniques being well known in the prior art and forming no part of the present invention which relates only to the generation of accoustical impulses in the water.

FIG. 2 shows the electrical source S connected through a suitable trigger T which operates to close, in effect, a switch to deliver the electrical energy from the source S directly across spaced electrodes 12 and 14, respectively. For instance, assume that the charge is delivered as a unidirectional pulse to the electrode 12 and that the electrode 14 functions as a return path. It is also convenient to assume that the electrode 12 is towed through the water ahead of the electrode 14 so that the wire W which is fed from the electrode 12 will be carried in the direction of the electrode 14 by the forward motion provided by the vessel V.

In the embodiment of FIG. 2, the wire feed means comprises a reel 16 onto which wire 18 is wound, the wire being pulled from the drum and through a tube 20 by means to be presently described. The term Wire as used herein refers to any filar or ribbon shaped material having good conductivity, for instance copper. Only a short length of wire need be fed to complete a circuit across the region R between the electrodes, just enough to ensure good contact with the electrode 14 before the trigger T is actuated to initiate the next shot.

The wire 18 is fed from the reel 16 through the tube 20 and the electrode 12 toward the electrode 14. The force causing the wire to unreel and pass through the tube 20 is provided by flowing water through the tube and out through an opening in the electrode 12. The tube 20 when grouped together with the transmission wires 22 and 24 comprise the cable C leading from the vessel V to the electrodes.

The leading end of the tube 20 is secured aboard the vessel V, and is connected to a T 26 into which sea water is pumped under considerable pressure through a pipe 28 from an entrance 30 below the water line. The pump 32 is driven by a motor 34 which in the practical embodiment of the present invention is an internal combustion engine. A pressure gauge 36 is provided to monitor the water feed. The right end of the T 26 is connected to the tube 20, but the left end of the T is nearly closed by a plug 38 having a tiny orifice 40 therethrough, which orifice is about two thousandths of an inch greater in diameter than the diameter of the wire 18. The practical embodiment currently in use employs #38 gauge wire passing through tube 20 which is a rubber hose of 4 internal diameter and feet in length. The water pressure at the gauge 36 is about 400 p.s.i., and the discharge at the trailing end ,of the hose is directed across the region between the electrodes. For the sake of simplicity, the water is pumped continuously through the tube 20, and therefore the system em loys a simple mechanical brake 42 which normally engages the reel and controls its unwinding to make it intermittent.

A timer-control K which may advantageously be operated by a clock motor actuates the trigger circuit T to periodically close the switch 10 to discharge the electrical energy through the electrodes, and after the energy has been discharged, the timer control circuit K briefly releases the brake 42 long enough to allow a short length of wire to exit from the electrode 12 and extend past the electrode 14 in contact therewith.

Considerable difficulty was experienced in providing satisfactory electrodes 12 and 14 having the desired rigid spacing in structure which could be towed stably, while at the same time providing electrode supports and structures which would not be eroded away at a prohibitive rate by the energy discharge. FIG. 3 shows a structure which has been found suitable and which includes two fiberglass bars 50 and 52 joined together by a pipe yoke 53 to which the fiberglass bars are attached by straps 54. The electrodes 12 and 14 straddle the fiberglass bars 52 and 50 to which they are respectively attached and thus tend to shield these supports from discharge erosion. Moreover, the electrodes are provided with rounded surfaces facing toward each other, which surfaces do not tend to eat away unduly rapidly as a result of the electrical discharge.

The leading electrode 12 is connected to the hose 20 and has a bore 21a extending through it and in communication with the bore of the hose 20. The wire 18 passes out through the opening 12a and is carried toward the electrode 14 by the slip stream of water flowing around the electrode 12 and out through its bore 12a. The latter electrode is streamlined, and its teardrop shaped surface 14a tends to cause the water to hug the wire against it according to well-known principles of laminar flow. The water jetting out of the bore 12a is directed in a way to improve the contact of the wire with the electrode 14.

During operation of the present apparatus, the timer control K releases the brake 42 for a moment sufficient to cause the wire to feed from the bore 12a in the elec' trode 12 against and preferably somewhat beyond the electrode 14. The brake 42 is soon set again to stop the unwinding of the wire 18 from the reel 16, and the electrical discharge is then initiated by the timer control K actuating the trigger T to close the switch 10. This entire cycle is repeated periodically as the vessel moves through the water in accordance with well-known seismic exploration techniques.

The present invention is not to be limited to the exact form shown in the drawing, for obviously changes can be made therein within the scope of the following claims.

We claim:

1. Apparatus for generating underwater accoustical impulses, comprising:

(a) electrode means immersed in the water in mutually opposed spaced relationship to provide a region therebetween which is large as compared with the area of the electrode means presented to the region;

(b) a source of wire;

(c) tubular means leading from said wire source to one electrode means and terminating in a conductive bore contacting said one electrode means and directed across the water in said region toward the opposed electrode means, the wire extending through the tubular means;

(d) means for pumping fluid through the tubular means and the bore under sufiicient pressure to form a jet stream operative to entrain the wire and pull it therethrough and to cause the entrained wire to be swept into contact with the opposed electrode means and form a metalized path across the water in the region; and

(e) a source of electrical energy connected across said opposed electrodes means, and having sufiicient energy when actuated to vaporize the wire in said path.

2. In apparatus as set forth in claim 1, said opposed electrode means being streamlined to have a teardrop cross-sectional shape and being oriented with the thicker portion of the teardrop shape facing toward said one electrode means, said opposed electrode means being located in said jet stream such that the water and wire pass around the latter electrode maens in laminar flow, which causes the wire to hug and contact the streamlined electrode surfaces.

3. In apparatus as set forth in claim 1, said electrode means including a towing yoke attached to support insulating bars spaced apart in the direction of towing and disposed normal thereto; and said electrode means comprising heavy metal electrodes attached to and straddling each bar, the electrodes having streamlined outer surfaces, and the thicker port-ions of the streamlined electrodes facing toward each other.

4. In apparatus as set forth in claim 3, said fluid comprising water; the leading electrode in said towing yoke having said bore extending therethrough facing the opposed electrode, and said tubular means being connected to discharge a jet of water and said entrained wire through the bore and against said following electrode.

5. In apparatus as set forth in claim 1, said apparatus comprising a marine survey system including a vessel having said sources aboard and having means for towing said electrode means behind the vessel and the towing means including said tubular means, said pumping means being aboard the vessel, a pipe coupling said pumping means with said tubular means in the vessel, the pipe having an orifice therethrough communicating with the tubular means and snuggly passing the wire from its source into the latter.

6. In apparatus as set forth in claim 1, timer control means connected to intermittently actuate said energy source; brake means controlling the pulling of the entrained wire through the tubular means, and the timer control means being connected to control the brake means to permit said pulling of the wire during intervals between said actuations of the energy source.

References Cited UNITED STATES PATENTS 3,251,027 5/1966 Huckabay et al. 3407 XR 3,268,028 8/1966 Miller 181-0.5 3,286,226 11/1966 Kearsley et al. 18l0.5

FOREIGN PATENTS 723,001 12/ 1965 Canada.

BENJAMIN A. BORCHELT, Primary Examiner.

W. KUJAWA, Assistant Examiner. 

